The invention relates to a diagnostic X-ray installation comprising an image intensifier television chain which includes an X-ray image intensifier, an optical coupler, a camera tube, a video amplifier and a monitor, and comprising a control loop for exposure control which includes a setpoint value transmitter, a control circuit for the formation of a control voltage for controlling the dose rate of the X-ray tube and a dominant field selection circuit which contains an evaluation circuit and a device for blanking out parts of the video signal for the formation of a dominant field.
In German Pat. No. 1,194,070, a diagnostic X-ray installation of this type is described. The video signal is supplied, in the control loop, to the evaluation circuit which effects an intergration of the signal. In a circuit arrangement the integrated signal is compared with a selected setpoint value. An electronic switch, controlled by the line-scanning frequency of the video signal, allows only the portion of the video signal to pass through which corresponds to the desired dominant field. The resulting partially blanked out signal is supplied to the control circuit which effects an adjustment of the radiographic values. A control of this type is comparatively sluggish. It can be employed only for fluoroscopy operation.
In the German OS No. 2,746,285 an installation for the suppression of picture noise is described in which several pictures are superimposed. If this installation is employed in a diagnostic X-ray installation for the suppression of quantum noise in the case of low dose rates, the described control cannot be applied since no control signal can be obtained from the superimposition of several pictures for the control of the brightness of individual pictures.
From the German Pat. No. 1,956,070, exposure control on the basis of the measurement of the photocathode current of the X-ray image intensifier is known. This control installation renders possible a rapid exposure control. However, since the inlet fluorescent screen of the X-ray image intensifier receives the entire radiograph, the photocathode current represents only the mean value of the image brightness, so that no dominant can be formed from this signal.